Need help estimating the AC size for my house

5 years ago

I've contacted multiple companies and all the estimates I received had either a 3 or a 2.5 ton AC, except for one who said my duct work cannot support more than 2 ton. So here I am asking for your expert opinion. I'll list of the details I think could be relevant, but if you need more details please let me know.

The house is a split-level w/ basement built in the 1970s. It faces north and all the windows have been replaced with double pane vinyl. Here are the measurements (in inches) and count.

North

70x78 - 1
70x70 - 1

South

70x45 - 1
70x40 - 2
70x34 - 1
34x85 - 1

East

46x40 - 1

West

70x78 - 1
70x45 - 1
34x40 - 2

The furnace is in the garage and there is space outside right behind the furnace to install the AC. It was installed last year and is a American Standard 2-stage variable-speed furnace (Model #AUD2B060A9V3VB). The return air duct which enters the furnace pops every time the second stage kicks in. The furnace sits on a 14" high base (return air duct?) and has 21" available on the top for an evaporator coil. I've been told it's barely enough. The return air duct going into the furnace and the duct going out of the furnace are both 24x6. The one and only return for the house is in the basement, is 18x8 and spans 2 joists (not sure if I am using this right but the return is not a metal duct but is instead the space between the ceiling of the garage and the floor of the first floor). The whole house is 1800 sq ft in size and has 11 registers (3 in the basement and 8 in the first floor).

Now the guy who suggested the 2 ton AC took measurements for the duct work and windows which no other person did. I don't know if it's a gimmick or if I should take his word for it because most online calculators are suggesting either a 2.5 or 3 ton. I'd appreciate any insight you folks could provide.

I am in the Seattle area.

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Comments (3)

tigerdunes
5 years ago
What size AC do you have now? Heating/cooling loads for basement area are small. What is typical average outside temp in the summers. I thought Seattle climate was relatively mild. sizing of your ductwork especially return needs to be addressed. Definitely not a 3 Ton condenser.

TD
PRO
Austin Air Companie
5 years ago
Your furnace is rated for 3 tons of air.

Whether you need 3 tons or not, you got it now. Your climate of Seattle is more moderate, so quite possible you don't need 3 tons of AC, probably don't need 60K BTU of heat either.

Especially if the duct work is popping when second stage heat engages.

The thing is if you put in a smaller unit and it doesn't keep up you can insulate more. If you put in a too large of unit, you replace the too larger unit there isn't no fix for over sized other than to rip it out and replace it again.

Because your climate is more mild, likely suggests your current furnace is over sized. But a guess on the internet, is just a guess. Nothing beats being there in person.... HVAC is an in person, on site job.

I service the Katy, Texas area.
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mike_home
5 years ago
Taking measurements and advising you your duct work can only handle air flow for 2 tons of cooling is not a gimmick. That is how is supposed to be done. A smaller AC unit means less profit for the installer. He is looking out for your best interest.

The summer design temperature for Seattle is about 82 degrees. Do you know how much insulation is in the attic? If it is old and less than R30 I suggest you spend money improving it. In your climate with average insulation the 2 ton size should be good, I would not go beyond 2.5 tons.

Your duct work is undersized and needs work. Using space between joists for a return is a terrible practice in my opinion. You need a return on the second floor. The one in the basement is not doing much when the basement door is closed and the fact it is only 18 X 8.

The winter design temperature for Seattle is about 27 degrees. I am surprised to hear your 60K (80% efficiency) furnace goes into the second stage. Does this happen all the time? What thermostat do you have? With a 2-stage thermostat I would not expect to the furnace to go into the second stage unless you set back the temperature several degrees at night. This will happen if the furnace going into the second stage based on a time delay set on the control board. This is something you should investigate.